In the provision of realistic joints, as used in prosthetic limbs, an important aspect in attempting to achieve a realistic movement is to provide a different operating characteristic to the joint when under load. Indeed, one of the more important characteristics of an artificial leg for achieving a natural-looking walking gait correspond with those of a so called stabilised knee, i.e. a knee which resists flexion when under load, that is when it is bearing at least some of the weight of the amputee. It can be said that the test is somewhat subjective if one were to view an amputee with an artificial leg, but mere observation is one of the best tests; a strange gait is generally immediately apparent; this can cause unwelcome stares to amputees, who when fitted with a pair of long trousers would otherwise not be noticed when in a crowd. Reference is made to literature associated with the Advanced Prosthetics course by J. Boender at Strathclyde University, which provides a detailed review of artificial knee joints.
Early prosthetic limb systems; dating from the 1950's were provided with friction brake devices. For example, in GB779087, when utilized in a knee joint application, there was a provided a shin and knee joint mechanism which included a drum fixed to the shin, with one or more bands connected to the thigh and embracing the drum so that the bands gripped the drum to lock the knee when the leg was bearing weight, actuating means associated with the shin and thigh operated to release the lock just before the foot left the ground in walking, with a connection between the shin and thigh permitting relative axial movement between the shin and thigh. In this device, however, an axial load on the limb produced a small rotation of the radius arm or arms causing the brake band or brake shoe to grip the drum and to resist knee flexion. Indeed, the resistance would frequently become so great such that the knee became automatically locked once sufficient load had been applied. Later devices were combined with a pneumatic piston and cylinder assembly which applied lower degrees of resistance to flexion and/or extension of the knee to control the motion of the shin during the swing phase.
In recent years, however, such friction-based systems—which required regular servicing and adjustment have been replaced by hydraulic dampers with external control, which provide resistance to flexion during a stance phase as well as a swing phase of operation by means of a piston and cylinder, assembly. Hydraulic artificial knees provide stability to the prosthesis when the patient's weight is borne on the prosthesis, and collapse must be prevented. To prevent collapse of a free artificial knee joint the joint must receive appropriate information to inform it of its required mode of function. The hydraulic knee joint operates by utilising a volume of incompressible fluid to the knee joint, whereby to provide mechanical stability.
One example of such an arrangement is the hydraulic “S-N-S” knee control system manufactured by Mauch Laboratories, Inc. In some situations, however, this system required an amputee to make a knee-extending movement before flexion could be initiated. Additional problems arose through external wear and through the fact that they require actuation, which is, of course dependent upon movement being regular. As is known, when walking, one will vary one's gait to go down stairs, to cross steps, to avoid obstructions and the like. In some cases mechanical switching of the valve will not be effected properly. U.S. Pat. No. 5,376,137 to Blatchford is an example of a weight activated knee joint with hydraulic amplification of weight application triggered pivotal movement, whilst U.S. Pat. No. 6,106,560 to Ultimate Knee teaches of a weight activated knee joint with mechanical amplification of weight application triggered pivotal movement.
Whilst recent hydraulic devices are believed to be much improved they are complex and costly to manufacture; they are manufactured to high tolerances. If mechanical external valve control is provided, then there will be problems as discussed above. Alternatively, electronic control and flow control valves can be provided—that are expensive to purchase and maintain—which enable amputees to walk with a pre-determined gait yet will not necessarily be reactive to uneven surfaces.
To assist in the understanding of the problems addressed by the present invention, reference shall now be made to standard testing techniques, as employed to assess the fitness of an amputee to walk with or without assistance. FIG. 1 is a graph that represents the maximum knee flexion angle in swing at various walking speeds for three known devices of prosthetic knee joints, (Source: “What are the benefits of the C-Leg?” (J. Kastner, R. Immervoll, H. Kristen & P. Wagner)). The graph shows a clear change of at least 10° in angular variation of knee flexion movement as an increase in walking speed for the computer controlled device yet a change of 20° over a speed change of 2 km/h for a known mechanically controlled device. The plot of knee flexion for a normal gait, i.e. for a person with the full use of their own limbs is shown for comparative purposes. It can be seen that the variation in knee flexion angle is more or less absent, for a control person, with full use of their own legs, despite an increase in speed to approximately double that of the initial speed. In simple terms, to the casual observer, the person wearing such artificial limbs would be seen to have an ungainly gait due to a delay in knee extension needed to prepare the limb for weight acceptance. In another study “User-adaptive control of a magneto-rheological prosthetic knee” (H. Herr and A. Wilkenfeld) which appeared in Industrial Robot: An International Journal Volume 30, Number 1, 2003, 42-55, where a computer controlled prosthesis—a Rheo Knee—was tested. A horizontal response was produced but the study was limited to below 5 km/hr limit, which is less than normal walking speeds.
Certain prostheses provide joints that use a weight activated safety mechanism which is energized by forces present in weight bearing, a weight bearing on the artificial limb, that can be detected and be made effective in implementing a change in resistance to limb pivoting. Such a movement can be the compression of one of the members that form a mechanical chain from the amputation stump to the ground, and which can be detected by strain gauges, by small amounts of telescopic deformations, or limited ranges of pivotal movements such as the knee or ankle and can typically be detected as relative displacements of two suitably chosen points. The pivotal movements can be amplified mechanically (including hydraulically) or electronically and/or can be made to be more sensitive to forces transmitted through a heel of an artificial foot as opposed to those input through a fore part of a foot.
Problems arising from the use of weight activated knee joint control mechanisms include the fact that the residual weight taken by the artificial limb on toe off can be inhibitive to the release of the weight-activated mechanism. Typically an apparatus or means is supplied to cause a threshold value of weight required to activate the knee stability. This threshold is easily overcome by force entry through the heel and is not easily overcome by force entry through the toe. Nevertheless, this threshold takes away from the ease of activation on heel strike, which subtracts from the total ease of use of the artificial limb, and low grades of attention will be needed at all times.
The same threshold also makes it difficult to find instant effortless stability of the knee, when extension after mid swing is incomplete the respective foot is susceptible of hitting the ground too early. This is particularly true when traversing rough pathways. Typically the weight-activation class of knee joints do not provide security against collapse in such conditions or in the event of accidental use.
One difficulty to be overcome by users of prosthetic lower limbs is that it is counter intuitive to place one's body weight onto the device to secure the same body weight against sudden collapse. This is not a problem in certain types of knee which default to a weight acceptance mode; the knee stability is deactivated by a mechanism that detects hyperextension of the knee joint, which typically goes together with a load on a fore-foot part of the prosthesis, but which can also be provided by voluntary hip femoral stump hip extension. Again there is a small movement that can inform the knee design of a required change of status, and also here various similar signal amplification means can be employed, i.e. mechanical, electronic, hydraulic.
Whereas the weight activated mechanisms have their weight application input parameter present throughout an activation period, the hyperextension that deactivates the knee stability mode is removed as soon as the joint commences its permitted free flexion to support the required free knee flexion for swing, as required in the use of a free knee, which requires the joint to have a memory, for a period as long as the knee is in free flexion mode, being a preceding condition of hyperextension. This period corresponds to the first half of the pendulum swing movement of the shank of the prosthesis relative to the thigh member. Additionally, the memory mode must be deactivated on knee extension in order to return the knee joint to its default state, where it must be ready to take an amputee's weight.
FIG. 2 details a switching function of the Mauch SNS prosthesis, wherein the memory function is determined by a mechanical arrangement (as opposed to an electronic timer arrangement, for example). Specifically, the memory comprises an eccentric toggle (H1), that resumes a gravitational neutral position unless prevented by an open state of a valve member (H3), which is energized by hydraulic pressure (H4) caused by a flex movement of the free knee. It is notable that this linkage is not instantaneous; release of the memory function takes a time frame independent of the time it takes to reapply weight on the prosthesis. In the event of an inadvertent early reapplication of weight on the prosthesis, the toggle would likely not be in a position to allow closure of the valve, which could be painful and perhaps cause an amputee to fall over following knee collapse in such circumstances. In effect, the memory of this prior teaching is continued for a longer period than desired whereby to cause at the very least a non-natural gait, with an increased likelihood of a fall occurring due to the time required for a change in state being far greater than desirable.
In the case of hip prostheses, it would be advantageous if the hip were furnished with a hydraulic apparatus which would immediately take a load on heel strike, and allow the patient to sink into the fluid of the hydraulic apparatus with such a speed that the socket remains substantially more in a position of a neutral pelvis.